The objective of the proposed project is to examine how visual and vocal social rhythmic entrainment scaffolds language development in typically developing infants and those at risk for autism spectrum disorder (ASD). ASD is a common and lifelong neurodevelopmental disorder characterized by impairments in social communication and repetitive behaviors. Yet, there is striking heterogeneity in language acquisition in children with ASD. Additionally, language delays in ASD are often a first concern for parents that their child is not developing as expected. Elucidating individual differences in language acquisition and promoting meaningful language development in ASD is of utmost importance because language abilities are linked with long-term outcome. In typically developing infants, rhythm plays a crucial role in social communication and language development. This is exemplified by caregivers? use of highly rhythmic infant-directed speech and singing to attract and maintain their infants? attention. Infants attend to rhythm at multiple levels including the acoustic speech signal (e.g., rhythmic stress cues) and the social interaction in which speech is embedded (e.g., rhythmically coordinated eye gaze, gestures, vocalizations). Individual differences in social rhythm sensitivity may be a marker of social attunement and serve as an important predictor of individual differences in language development in ASD. This R21 extends our prior work demonstrating rhythmically entrained eye gaze in typical infants and toddlers to infants at-risk for ASD by examining rhythmic social visual engagement using novel eye-tracking paradigms (Aim 1) and rhythmic social vocal engagement based on an innovative marker of speech rhythms (Aim 2). Moreover, we propose to examine whether visual and vocal social entrainment predict individual differences in language acquisition (Aim 3). This research is well aligned with NIDCD?s focus on language acquisition in children with ASD. These findings are expected to provide preliminary data to guide future longitudinal investigations of social entrainment and language development in ASD. This research may lead to the development of biomarkers of rhythmic entrainment that may inform early ASD assessment and diagnosis, as well as intervention strategies that, if warranted, include rhythm and timing of social interactions as one aspect of comprehensive language intervention for these children.